1. Field of the Invention
This invention relates to an electric vehicle control apparatus which is connected to a plurality of motors provided in an electric vehicle and which controls the speeds of the motors individually.
2. Description of the Related Art
An inverter apparatus has been conventionally used as an electric vehicle control apparatus. Traditionally, in this conventional method only one inverter apparatus is used to control a plurality of motors. However, recently, consideration has been given to the fact that the diameters of a wheels powering the electric vehicle are changed by friction which is generated between the rail and the wheels over time. The changes in diameter are different for each wheel. Hence it has been understood that the wheels needed to be controlled individually. Accordingly, a method has been employed in which a plurality of inverter devices are connected respectively to each motor so that they may be individually controlled. Such a conventional individual control method will now be described in relation to FIG. 1.
FIG. 1 shows the control structure of an electric motor vehicle control apparatus using the conventional individual control method. In FIG. 1, inverter apparatuses IV.sub.1 through IV.sub.4 include main circuits T.sub.l through T.sub.4 and control circuits C'.sub.1 through C'.sub.4, respectively. Electric power from a pantograph 1 is provided to main circuits T.sub.1 through T.sub.4 through a breaker 2. Induction motors M.sub.1 through M.sub.4 are controlled individually in response to the outputs from the main circuits T.sub.1 through T.sub.4.
Control circuits C'.sub.1 through C'.sub.4 receive a notch command n (speed increase/decrease) and a direction command F/R (forward/reverse) from a main controller 3. Further, the control circuits C'.sub.1 through C'.sub.4 are provided with feedback current signals i.sub.1 through i.sub.4 from current detectors CT.sub.1 through CT.sub.4 and rotor (or wheel) speed signals s.sub.1 through s.sub.4 from speed detectors TG.sub.1 through TG.sub.4. The control circuits C'.sub.1 through C'.sub.4 produce gate signals g.sub.1 through g.sub.4 in response to the notch command n, direction command F/R, current signals i.sub.1 through i.sub.4 and speed signals s.sub.1 through s.sub.4. The gate signals are provided to the main circuits T.sub.1 through T.sub.4 in order to control the output current.
In the above individual control method, when there is a difference between the diameters of the wheels driven by the respective motors, the motors are separately controlled on each wheel and the torque of the motors can, therefore, be individually controlled. Since readhesion control can be accomplished when either a slip or slide (i.e., a deficiency in friction) occurs between the rail and the wheel, performance of the electric motor vehicle as a whole is improved. The term readhesion control refers to control which causes the wheel which is slipping or sliding to regain sufficient contact with the rail.
The induction motor can be controlled by reducing the "slip" (i.e. synchronized speed minus the actual rotor speed) in the induction motor. Since this "slip" produces a power loss associated with the induction motor, as the "slip" is reduced, the efficiency of the motor is improved.
When a slip or slide occurs between the rail and wheel, it is important that the condition be detected as soon as possible so that readhesion control can be applied. However, in the conventional individual control method, only the speed signal of the wheel on which a slip is to be detected is used as a source of information to detect when a slip (or slide) occurs. Utilizing only a single speed parameter to detect the condition of a slip (or slide) limits the ability to obtain maximum control.